Aluminium metal is produced via the electrochemical dissociation of alumina dissolved in a fluoride melt consisting of AlF3 and NaF known as cryolite (3NaF. AlF3). The cell reaction involves several steps (see F Habashi: A Handbook of Extraction Metallurgy, vol. 3, VCH, Berlin) and relies on the use of carbon anodes and cathodes. To illustrate the need for a consumable carbon anode, a simplified description of the cell reaction is
                    1        2            ⁢                          ⁢              Al        2            ⁢              O        3            ⁢                          ⁢              {        dissolved        }              +                  3        4            ⁢              C        ⁡                  (          s          )                      =            Al      ⁡              (        l        )              +                  3        4            ⁢              CO        2            
The combustion of carbon is necessary to maintain the temperature of the molten aluminium and cryolite bath which moderates the electrical energy consumption of the cell. In the cell, the power consumption for making aluminium is of the order of 6.3 kWh/kg which is equivalent to 2.1 V and represents 50% of the total energy consumption of the cell. The remaining 50% (or 2.1 V) of the total energy consumption maintains the cell temperature in the face of heat losses (and is equivalent to 6.3 kWh/kg for making aluminium metal). For each tonne of aluminium metal produced, 333 kg of carbon is oxidised at the anode to carbon dioxide gas which escapes into the atmosphere. The evolution of carbon dioxide is one of the main sources of greenhouse gas emission in the aluminium industry.
Periodically (eg monthly) the carbon electrode is replaced with a new one. During this change over period, the electrolyte in the bath becomes under saturated and reacts with carbon to produce small concentrations of perfluorocarbon (PFC) gases. Moreover the presence of fluoride salt melt in the Al-electrolytic cell and the large current surge during cell operation lead to decomposition of fluoride salts into reactive forms of fluorine gas which readily react with carbon present in the electrodes to generate PFCs. PFCs also form during anode effect. When the PFCs escape into the atmosphere, they contribute to ozone depletion. PFCs also pose a major health risk to plant workers.
The manufacture of carbon electrodes uses petroleum products which decompose and release hydrocarbon based greenhouse gases. The processing and manufacturing route for electrodes is quite complex and time-consuming. In the lengthy process, the material is prebaked and fired for graphitization at 3000° C. for 1 month. A large volume of greenhouse gases (eg methane, sulphur and sulphur dioxide) is emitted during anode fabrication. The costs of energy consumption for a carbon anode is as large as the production metal. Coal tar pitch is used in making Soderberg anodes and during this process SO2 forms and contributes to environmental pollution. 11.5 mT of coke for making carbon anodes is consumed globally.
Global aluminium companies have targets to reduce the emission of greenhouse gases and ozone-depleting PFCs. In North America, the major aluminium metal producers have agreed to consider replacing carbon-based electrodes with new non-consumable/inert electrodes.
Most inert electrodes developed to date are based on ceramic powder and cermet-based technologies. ALCOA has successfully demonstrated the use of NiO.Fe2O3-based cermets with a noble metal such as silver and copper for enhancing the electronic conductivity of the cermet electrodes (see U.S. Pat. No. 5,865,980). Since the cermets are made via the ceramic powder fabrication technique, there is apparently a cost implication compared to molten metal melting and casting techniques. Although nickel ferrites have both ionic and electronic conductivities, the major enhancement in the electronic conductivity arises from the presence of the noble metallic phases dispersed in the nickel-ferrite matrix. However the fabrication of ferrite anodes is via ceramic processing and requires firing and sintering above 1100° C. for several days.
For many years, titanium diboride powders have been used for making ceramic electrodes for producing molten aluminium (see U.S. Pat. No. 4,929,328). The diborides exhibit high-temperature electrical resistivity of 14 μohm cm and thermal conductivity of 59 W m−2 K−1. The sintered materials also exhibit high oxidation and corrosion resistance. TiB2 has a high melting point and so there is an inherent cost for processing and sintering ceramic powders. Adding alumina in the matrix for reducing the processing and sintering temperatures compromises the conductivity of TiB2 and its composites. The composite can also be fabricated by making a partially sintered material using the self-heating high-temperature synthesis (SHS) of TiB2 and alumina. There has been also some research and development activity in processing copper-nickel, copper-nickel-iron and copper-based cermets for electrode materials (see U.S. Pat. No. 6,126,799, U.S. Pat. No. 6,030,518 and D R Sadoway: “Inert Anodes for Hall-Héraoult cell—the ultimate materials challenge”, J Metals, vol. 53, May 2001, pp. 34-35). However there appears to be some reliability issues for such electrode materials at high-temperatures due to the high solubility of copper in liquid and solid aluminium which may reduce the structural performance of the copper-based cermets.
The present invention is based on the recognition that certain Al-M-Cu based alloys exhibit high-temperature strength, corrosion resistance and electrical conductivity without major resistive heat loss and so can be exploited as an inert electrode, in particular as an inert electrode to replace carbon anodes in a Hall-Héraoult cell for extraction of reactive metals such as Al, Ti, Nb, Ta, Cr and rare-earth metals.
Thus viewed from one aspect the present invention provides an electrode (eg an anode) composed of an Al-M-Cu based alloy comprising an intermetallic phase of formula:AlxMyCuz wherein:
M denotes one or more metallic elements;
x is an integer in the range 1 to 5;
y is an integer being 1 or 2; and
z is an integer being 1 or 2.
The electrical resistivity of embodiments of the electrode of the invention was found to decrease as a function of temperature and illustrates the usefulness of the ordered high-temperature alloy as an inert electrode. The desirable electronic conductivity arises due to the presence of metallic copper which has the added advantage that it is much cheaper than alternatives such as silver and gold. By way of example the electrode of the invention performs well as an anode an alumina-saturated cryolite bath at 850° C.
The Al-M-Cu based alloy may be substantially monophasic or multiphasic. Preferably the intermetallic phase is present in the Al-M-Cu based alloy in an amount of 50 wt % or more (eg in the range 50 to 99 wt %). Preferably the Al-M-Cu based alloy further comprises an ordered high-temperature intermetallic phase of M with aluminium, particularly preferably Al3M. Other intermetallic phases may be present.
In a preferred embodiment, the Al-M-Cu based alloy is substantially free of CuAl2. This is advantageous because CuAl2 has a tendency to melt at the elevated temperatures which are deployed typically in metal extraction (eg 750° C. for aluminium extraction). Preferably CuAl2 is complexed.
In a preferred embodiment, the Al-M-Cu based alloy falls other than on the M poor side of the tie line joining Al3M and MCu4 (eg on the M rich side of the tie line joining Al3M and MCu4).
In a preferred embodiment, the Al-M-Cu based alloy comprises an intermetallic phase falling on or near to the tie line joining Al3M and MCu4.
In a preferred embodiment, the Al-M-Cu based alloy falls other than on the M poor side of the tie line joining Al3M and AlMCu2 (eg on the M rich side of the tie line joining Al3M and AlMCu2).
In a preferred embodiment, the Al-M-Cu based alloy comprises an intermetallic phase falling on or near to the tie line joining Al3M and AlMCu2.
In a preferred embodiment, the Al-M-Cu based alloy falls other than on the M poor side of the ξ, Al5M2Cu, MAlCu2 and β-MCu4 phase tie line (wherein ξ is a phase falling between Al3Ti and Al2Ti with 3 at % or less of Cu (eg 2-3 at % Cu)).
In a preferred embodiment, the Al-M-Cu based alloy comprises an intermetallic phase falling on or near to the ξ, Al5M2Cu, MAlCu2 and β-MCu4 phase tie line.
Preferably the intermetallic phase is Al5M2Cu. Particularly preferably the Al-M-Cu based alloy further comprises Al3M.
Preferably the intermetallic phase is MAlCu2. Particularly preferably the Al-M-Cu based alloy further comprises β-MCu4.
The electrode may be composed of a homogenous, partially homogenous or non-homogeneous Al-M-Cu based alloy.
In a preferred embodiment, the electrode comprises a passivating layer. Preferably the passivating layer withstands electrode oxidation in anodic conditions.
In a preferred embodiment, M is a single metallic element. The single metallic element is preferably Ti.
In an alternative preferred embodiment, M is a plurality (eg two, three, four, five, six or seven) of metallic elements. In this embodiment, a first metallic element is preferably Ti. Typically the first metallic element of the plurality of metallic elements is present in a substantially higher amount than the other metallic elements of the plurality of metallic elements. Each of the other metallic elements may be present in a trace amount. Each of the other metallic elements may be a dopant. Each of the other metallic elements may substitute Al, Cu or the first metallic element. The presence of the other metallic elements may improve the high-temperature stability of the alloy (eg from 1200° C. to 1400° C.).
In a preferred embodiment, M is a pair of metallic elements. In this embodiment, a first metallic element is preferably Ti. Typically the first metallic element of the pair of metallic elements is present in a substantially higher amount than a second metallic element of the pair of metallic elements (eg in a weight ratio of about 9:1). The second metallic element may be present in a trace amount. The second metallic element may be a dopant. The second metallic element may substitute Al, Cu or the first metallic element. The presence of a second metallic element may improve the high-temperature stability of the alloy (eg from 1200° C. to 1400° C.).
Preferably the pair of metallic elements have similar atomic radii. Preferably the atomic radius of the second metallic element is similar to the atomic radius of Cu. Preferably the atomic radius of the second metallic element is similar to the atomic radius of Al.
In a preferred embodiment, M is one or more of the group consisting of group B transition metal elements (eg first row group B transition metal elements) and lanthanide elements. Preferably M is one or more group IVB, VB, VIB, VIIB or VIIIB transition metal elements, particularly preferably one or more group IVB, VIIB or VIIIB transition metal elements.
In a preferred embodiment, M is one or more metallic elements of valency II, III, IV or V, preferably II, III or IV.
In a preferred embodiment, M is one or more metallic elements selected from the group consisting of Ti, Zr, Cr, Nb, V, Co, Ta, Fe, Ni, La and Mn. In a particularly preferred embodiment, M is one or more metallic elements selected from the group consisting of Ti, Fe, Cr and Ni.
Preferably M is or includes a metallic element capable of reducing the tendency of CuAl2 towards grain boundary segregation at an elevated temperature. In this embodiment, the metallic element capable of reducing the tendency of CuAl2 towards grain boundary segregation at an elevated temperature may be the second metallic element of a plurality (eg a pair) of metallic elements. Particularly preferably M is or includes a metallic element capable of forming a complex with CuAl2. Preferred metallic elements for this purpose are selected from the group consisting of Fe, Ni and Cr, particularly preferably Ni and Fe, especially preferably Ni.
Preferably M is or includes a metallic element capable of reducing the tendency of the first metallic element or Cu to dissolve in molten extractant. In this embodiment, the metallic element may be the second metallic element of a plurality (eg a pair) of metallic elements. Preferred metallic elements for this purpose are selected from the group consisting of Fe, Ni, Co, Mn and Cr, particularly preferably the group consisting of Fe and Ni (optionally together with Cr).
Preferably M is or includes a metallic element capable of promoting the passivation of the surface of the electrode (eg anode) in the presence of a molten electrolyte. For this purpose, the metallic element may form or stabilise an oxide film. In this embodiment, the metallic element may be the second metallic element of a plurality (eg a pair) of metallic elements. Preferred metallic elements for this purpose are selected from the group consisting of Fe, Ni and Cr. Particularly preferably M is Ti, Fe, Ni and Cr in which the formation of a combination of oxides such as iron oxides, chromium oxides, nickel oxides and alumina advantageously promotes passivation.
Preferably M is or includes a metallic element selected from the group consisting of Zr, Nb and V. Particularly preferred is V or Nb. These second metallic elements are advantageously strong intermetallic formers. In this embodiment, the metallic element is the second metallic element of a plurality (eg a pair) of metallic elements.
Preferably M is or includes a metallic element capable of forming an ordered high-temperature intermetallic phase with aluminium metal. Particularly preferably M is or includes a metallic element capable of forming Al3M.
Preferably M is or includes Ti. A titanium containing alloy typically has electrical resistivity in the range 3 to 15 μohm cm at room temperature.
Preferably the intermetallic phase is Al5Ti2Cu. Particularly preferably the Al—Ti—Cu based alloy further comprises Al3Ti.
Preferably the intermetallic phase is TiAlCu2. Particularly preferably the Al—Ti—Cu based alloy further comprises β-TiCu4.
In a preferred embodiment, M is or includes Ti and a second metallic element selected from the group consisting of Fe, Cr, Ni, V, La, Nb and Zr, preferably the group consisting of Fe, Cr and Ni. The second metallic element advantageously serves to enhance high-temperature stability of the Al—Ti—Cu phases.
The electrode of the invention may be composed of an Al-M-Cu based alloy obtainable by processing a mixture of 35 atomic % Al or more (preferably 50 atomic % Al or more), 35 atomic % M or more (wherein M is a first metallic element as hereinbefore defined) and a balance of Cu and optionally M′ (wherein M′ is one or more additional metallic elements as hereinbefore defined).
In a preferred embodiment, the electrode of the invention is composed of an Al-M-Cu based alloy obtainable by processing a mixture of (65+x) atomic % Al, (20+y) atomic % M (wherein M is a first metallic element as hereinbefore defined) and (15−x−y) atomic % Cu, optionally together with z atomic % of M′ (wherein M′ is one or more additional metallic elements as hereinbefore defined) wherein M′ substitutes Cu, Al or M.
In this embodiment, the alloy may be obtainable by casting, preferably in an oxygen deficient atmosphere (eg an inert atmosphere). For example, a mixture may be melted in an argon-arc furnace under an atmosphere of argon gas and then solidified in an argon atmosphere. Alternatively in this embodiment, the alloy may be obtainable by flux-assisted melting. The electrode may be processed in near-net shape eg a finished square-shape rod.
In a preferred embodiment, the electrode of the invention is at least as conducting at elevated temperature (eg at 900° C.) as a carbon electrode.
In a preferred embodiment, the electrode of the invention exhibits good thermal conductivity.
In a preferred embodiment, the electrode of the invention is electrochemically stable (eg is substantially non-soluble in the electrolyte). In a preferred embodiment, the electrode of the invention is resistant to oxidation and corrosion at high temperatures.
In a preferred embodiment, the electrode of the invention exhibits good high-temperature strength, thermal shock and thermal and electrical fatigue resistance.
In a preferred embodiment, the electrode of the invention is wettable by a molten metal-containing source from which it is desired to extract metal (eg aluminium) whereby to reduce cathode resistance.
The electrode will generally be non-toxic and non-carcinogenic (and not lead to the generation of toxic or carcinogenic materials). The electrode may be recyclable. The electrode may be safely disposable.
It is quite well known within the aluminium industry that the Al3Ti phase can be dispersed via the reactive melting of aluminium metal in the presence of K2TiF6. The reaction between molten aluminium and K2TiF6 yields a mixture of Al3Ti and aluminium metal. This technique has however been only used to make binary Al—Ti alloys with less than 1-2 wt % Ti for which the processing temperature is between 750° C. and 850° C.
Viewed from a further aspect the present invention provides a process for preparing an Al-M-Cu based alloy as hereinbefore defined comprising:
(a) adding an alkali fluorometallate flux to a source of Cu and a source of Al.
In accordance with the process of the invention, the presence of fluorine (eg in a fluorine bath) advantageously reduces hydrogen solubility in the Al-M-Cu liquid to yield a porosity-free cast structure which would otherwise have a higher resistive loss due to a high volume of pores.
The alkali fluorometallate may be a potassium or sodium alkali fluorometallate (eg fluorotitanate) salt.
The source of Cu and source of Al may be a molten Al—Cu alloy.
In a preferred embodiment, step (a) is carried out in an oxygen deficient atmosphere (eg an inert atmosphere such as argon or nitrogen).
In a preferred embodiment, the process further comprises:
(b) annealing the Al-M-Cu cast alloy from step (a).
Step (b) may be carried out in an oxygen deficient atmosphere (eg an inert atmosphere such as argon or nitrogen) at temperatures typically in the range 600-1000° C. (eg about 800° C.). Step (b) serves to eliminate deleterious phases such as Al2Cu and other low melting point inhomogeneities.
Step (b) may be preceded or succeeded by (c) the formation (eg coating) of an oxide layer on the Al-M-Cu surface. The oxide layer is preferably a mixed oxide layer containing alumina, iron oxide, nickel oxide and optionally chromium oxide. Step (c) may be carried out at an elevated temperature. The oxide layer may be formed from a slurry of mixed oxides which may be applied to the cast alloy before step (b) or be subjected to a separate heating step. By way of example, a preferred slurry is a 50:50 by volume water/ethyl alcohol comprising 35-45 mol % Fe2O3, 30-45 mol % NiO, 10-20 mol % alumina and 0-5 mol % Cr2O3.
Viewed from a yet further aspect the present invention provides a method for extracting a reactive metal from a reactive metal-containing source comprising:                electrolytically contacting an electrode composed of an Al-M-Cu based alloy with the reactive metal-containing source.        
The electrode may be as hereinbefore defined for the first aspect of the invention. The reactive metal may be selected from the group consisting of Al, Ti, Nb, Ta, Cr and rare-earth metals (eg lanthanides or actinides). Preferred is Al.
Preferably the reactive metal-containing source is a molten bath, particularly preferably a molten bath containing reactive metal oxide. For the extraction of aluminium, the molten bath is alumina-containing, particularly preferably alumina-saturated, especially preferably is an alumina-saturated cryolite flux. Preferably the cryolite flux comprises sodium-containing potassium cryolite (eg sodium-containing 3KF.AlF3 such as K3AlF6—Na3AlF6) weight ratio of NaF to AlF3 in the sodium-containing potassium cryolite may be in the range to 1:1.5 to 1:2.
In a preferred embodiment, KBF4 is present in the cryolite flux. The presence of KBF4 dramatically improves the wettability of an electrode composed of an Al-M-Cu alloy.
Preferably alloy comprises a passivating layer which prevents oxidation under anodic conditions.
Viewed from a still yet further aspect the present invention provides the use of an Al-M-Cu based alloy as an anode in an electrolytic cell.
Preferably the Al-M-Cu based alloy in this aspect of the invention is as hereinbefore defined.
Viewed from an even still yet further aspect the present invention provides an electrolytic cell comprising an electrode as hereinbefore defined.